The present invention relates to a process for the preparation of paper-backed adhesive tapes which excel in the widthwise strength and is easily torn in the widthwise direction even by hand.
By way of example, reference is made to a corrugated box. A variety of adhesive tapes are used for sealing the flaps of a corrugated box. Among others, a typical adhesive tape is a cloth-backed adhesive tape.
The cloth-backed adhesive tape excels in the widthwise strength and possesses a so-called "widthwise tearing property" that permits easy tearing of the tape at a right angle to the lengthwise direction by hand but without using cutters or the like means. Comparing to the paper-backed tape of which backing is coated with silicone as release agent, the cloth-backed tape is able to lap-seal and the surface of the tape exposed to the atmosphere (hereinafter referred simply to as "the tape back") possesses a great slip resistance, so that when several corrugated boxed each being sealed at its flaps with such tapes are placed with one on top of another, they are very difficult to disjoint. In addition, the tape back sufficiently receives a kind of oily marking ink, e.g., sold under the trade mark "Magic Ink", and permits relatively easy unwinding. Thus, the cloth-backed adhesive tape has a number of advantages; however, it offers some proplems from an economical point of view.
Another representative tape is a kraft paper-backed adhesive tape. This type of an adhesive tape has some advantages that it is inexpensive as compared with the above-mentioned cloth-backed adhesive tape and is very easy to unwind. With this tape, however, difficulties are encountered in packaging of articles since it is difficult for an user to tear linearly the tape at an angle normal to the lengthwise direction thereof. In addition, this tape is disadvantageously in that it is not able to lap-seal, and its back possesses a considerably low slip resistance, so that when several corrugated boxed each being sealed at its flaps are placed with one on top of another, they are apt to disjoint, and its back exhibits a poor ink-receptibel property.
It should be noted that the term "lap-sealing property" used hereinafter indicates that the tape is able to lap-seal.
Attempts have been made at providing the properties inherent in the cloth-backed adhesive tape to the kraft paper-backed tape by scoring or perforating the tape in the widthwise direction such that it can linearly be torn along said score or perforation. However, when perforating the tape in the widthwise direction, the tape merely tears along the perforation which then results in a drop of the tape strength in the longitudinal, i.e., lengthwise direction. Thus, such attempts cause another defect and produce no satisfactory results.
Still another type of an adhesive tape supported on a variety of plastic films is disadvantageous in that it is apt to tear when there are projections or protrusions on an application surface. It is also difficult to tear the tape at the widthwise direction. No application of suitable release treatment results in difficulties in the unwind of the tape. Furthermore, it is difficult to pull this tape by a force applied to its end due to its thin thickness.
If the plastic film used is sufficiently stretched in the direction normal to the lengthwise direction, then the obtained adhesive tape is easy to tear in the widthwise, but exhibits a very poor lengthwise strength.
In what follows, reference will be made to the paper-backed tapes having a part of the characteristics of the aforesaid cloth-backed adhesive tapes and capable of being prepared at a low cost. Such tapes include a paper-backed adhesive tape prepared by laminating a polyethylene film oriented in the widthwise direction (The film thus obtained is hereinafter reffered to as the widthwise stretched film.) on one surface of a backing paper and coating a pressure-sensitive adhesive on the opposite surface of the backing paper. In effect, this adhesive tape can be prepared according to the teaching of Japanese Patent Application laid open for public inspection under No. 81543/1978.
However, unless the tape back is subjected to suitable release treatment or the bond strength of the pressure-sensitive adhesive is allowed to drop to an extremely low level, then said tape is so unsatisfactorily unwindable that the tape itself undergoes destruction due to delamination. When neither sufficient delamination strength nor suitable release treatment is applied to the backing paper, similar difficulties are encountered as experienced in Japanese Utility Model Publication Nos. 35589/1978 and 35591/1978.
The release treatment gives rises to decreases in the slip resistance, ink-receptible property, lap-sealing property and the like. When use is made of a pressure-sensitive adhesive having a limited bond strength, difficulties are encountered in sealing of, for instance, a corrugated box; the corrugated box is so insufficiently sealed that the sealed portions will be unsealed later.
Still another type of a paper-backed adhesive tape is proposed so as to obviate the disadvantages of said kraft paper-backed adhesive tape. Such a tape can be prepared by extruding and coating polyethylene on one surface of a backing paper at an extrusion temperature of 250.degree. to 290.degree. C. prevailing at the outlets of die lips and coating a pressure-sensitive adhesive composed mainly of polyacrylic ester on the opposite surface of the backing paper according to the method disclosed in Japanese Patent Publication No. 20205/1976. However, difficulties are still encountered when tearing this tape in the widthwise direction.
Various attempts have been made to propose adhesive tapes which have all the advantages of the prior art cloth-backed adhesive tape, and which can be prepared in a easy and inexpensive manner as compared therewith; however, satisfactory results are not still obtained.